Listen to the briefing: CHARLESTON, W.Va. -- The April 5 explosion at Massey Energy's Upper Big Branch Mine was so large and powerful that it ripped through more than 2 1/2 miles of underground tunnels "in an instant," leaving debris, soot and roof falls that continue to hamper investigation teams, a special adviser to Gov. Joe Manchin said Wednesday."That's a tremendous amount of energy released relatively quickly, and it spread in a large area very quickly," said Davitt McAteer, a former top federal mine safety regulator leading an independent probe for Manchin. "All of this suggests to us that the forces at work are complicated, are severe and are large."McAteer said investigators have questioned more than 125 witnesses, but still have 100 or more interviews to go. Underground evidence collection did not begin until late June, and crews probably need several more months to complete that work, McAteer said.And, McAteer revealed, investigators were able to examine the Raleigh County mine's longwall section only Tuesday, and still haven't been able to visit two nearby collections of tunnels that are other likely spots for the explosion to have begun. Along with damage from the explosion itself, parts of the mine have filled with water since the blast and need to be pumped dry before investigators can examine them."It's slow," McAteer said of the probe's progress. "That's not comforting to the families. We know that. But we only have one chance to do this. We want to do it right."McAteer said he had some hope that his independent report might be done before year's end, but separate reports from the U.S. Mine Safety and Health Administration and the state Office of Miners Health, Safety and Training aren't likely to be completed by then, officials said.Such timing isn't necessarily that unusual in major coal-mining disasters. For example, MSHA did not issue its report on the Jan. 2, 2006, Sago Mine explosion until May 2007.McAteer discussed his independent review of Upper Big Branch Wednesday during a Capitol press conference, marking the first time any government investigators have briefed the media on their progress in determining the cause of the worst U.S. coal-mining disaster in 40 years. Twenty-nine miners died in the April 5 explosion, which experts believe likely involved an ignition of methane gas made much worse by a buildup of highly explosive coal dust in the Massey mine.Manchin asked McAteer to conduct a review similar to those he performed after Sago and the January 2006 Aracoma Mine fire, with a focus on preventing future deaths and determining if existing laws are strong enough or weren't properly enforced.McAteer contrasted those efforts to investigations by MSHA and state regulators, which aim to find out if specific violations by Massey caused the disaster. Federal prosecutors and FBI agents are also conducting a separate investigation of the disaster, looking for potential criminal wrongdoing involving Upper Big Branch citations that date back more than four years."Our whole philosophy is separate," McAteer said. "Our whole purpose is separate."Already, he believes he's seen things at Upper Big Branch that confirm his findings after Sago and Aracoma that the mining industry hasn't moved as quickly to deploy mine safety technology improvement as it has mine production enhancements. He cited, for example, the fact that pre-shift safety records are still written by hand, rather than maintained in computer databases that can be more easily used to monitor mine conditions.McAteer said his team is continuing to investigate what -- if any -- steps were taken by Massey and MSHA to prevent a recurrence of two "methane outbursts" from the mine floor at Upper Big Branch in 2003 and 2004.In response to repeated questions on this point, McAteer said investigators have not yet interviewed all of the witnesses who would have information about the safety examinations conducted immediately prior to the work shift when the explosion occurred, and therefore could not say what those examinations revealed.Reach Ken Ward Jr. at email@example.com or 304-348-1702.